A citadel is a fortress protecting a town, sometimes incorporating a castle. The term derives from the same Latin root as the word "city", civis, meaning citizen.

In a fortification with bastions, the citadel is the strongest part of the system, sometimes well inside the outer walls and bastions, but often forming part of the outer wall for the sake of economy. It is positioned to be the last line of defence should the enemy breach the other components of the fortification system. A citadel is also a term of the third part of a medieval castle, with higher walls than the rest. It was to be the last line of defence before the keep itself.

The oldest known structures which have served as citadels were built by the Indus Valley Civilization, where the citadel represented a centralised authority. The main citadel in Indus Valley was almost 12 meters tall. The purpose of these structures, however, remains debated. Though the structures found in the ruins of Mohenjo-daro were walled, it is far from clear that these structures were defensive against enemy attacks. Rather, they may have been built to divert flood waters.

In ancient Greece, the Acropolis (literally: "peak of the city"), placed on a commanding eminence, was important in the life of the people, serving as a refuge and stronghold in peril and containing military and food supplies, the shrine of the god and a royal palace. The most well-known is the Acropolis of Athens, but nearly every Greek city-state had one - the Acrocorinth famed as a particularly strong fortress. In a much later period, when Greece was ruled by the Latin Empire, the same strongpoints were used by the new feudal rulers for much the same purposes.